1. Field of the Invention
This invention relates to a camera module, and more specifically to a camera module that can be used in small-sized electronic apparatuses such as digital cameras, camera-equipped cellular phones and the like.
2. Description of the Prior Art
A camera module having a function of displacing a lens unit in a direction of an optical axis of the lens unit is used in relatively small-sized digital cameras, camera-equipped cellular phones and the like. This function is used for providing an autofocus function and/or zoom function and the like, and is achieved by an interaction between a magnetic field generated by an electrical current flowing in a coil and a magnetic field generated by a yoke and magnets provided on the yoke.
Such a camera module includes a barrel which holds the lens unit, a holder which houses the barrel therein, and a pair of leaf springs for supporting the holder so that the holder is displaceable in a direction of an optical axis of the lens unit.
FIG. 5 is an exploded perspective view of an autofocus actuator assembly 100 used in a conventional camera module for displacing a lens unit (not shown) in a direction of an optical axis thereof. It is to be noted that an imaging element (not shown) is provided below a base 111 of the actuator assembly 100.
A holder 108 that houses a barrel (not shown) holding the lens unit is provided between a cover 101 and the base 111 so that the holder 108 is displaceable in a direction of an optical axis of the lens unit together with the lens unit.
A ring-shaped inner annular portion 104b of an upper leaf spring 104 and a ring-shaped inner annular portion 110b of a lower leaf spring 110 are attached to the upper and lower cylindrical portions of the holder 108, respectively. Further, a ring-shaped outer annular portion 104a of the upper leaf spring 104 is attached to a top surface of a yoke 105 which is mounted on the base 111 and a ring-shaped outer annular portion 110a of the lower leaf spring 110 is attached to the base 111, respectively.
Three bridge portions 104c are coupled between the outer annular portion 104a and the inner annular portion 104b of the upper leaf spring 104. As is the same with the upper leaf spring 104, three bridge portions 110c are coupled between the outer annular portion 110a and the inner annular portion 110b of the upper leaf spring 110. By resilient deformation of the respective three bridge portions 104c and 110c, the holder 108 can be displaced in a direction of an optical axis of the lens unit.
A plurality of magnets 106 are provided on the yoke 105 so as to produce a magnetic field. The yoke 105 has an axial bore 105a for receiving the holder 108. Further, a coil 107 is provided around an outer periphery of the holder 108 so that the coil 107 is disposed in the magnetic field produced by the magnets and the yoke 105 in a state that the holder 108 is received in the axial bore 105a. By supplying a current to the coil 107 to generate a magnetic field, the holder 108 can be displaced in a direction of an optical axis of the lens unit by a driving force generated by the interaction of the magnetic fields. In this regard, it is to be noted that a component denoted by the reference numeral 103 is a flexible printed circuit board used for supplying a current to the coil 107, a component denoted by the reference numeral 102 is a stopper arranged above the upper surface of the inner annular portion of the upper leaf spring 104, and a component donated by the reference numeral 109 is a plate provided between the lower leaf spring 110 and the bottom surface of the yoke 105.
FIG. 6 is a perspective view of the upper leaf spring 104 and the yoke 105. As described above, the upper leaf spring 104 includes the outer annular portion 104a and the inner annular portion 104b arranged inside the outer annular portion 104a through an annular spacing 104f and having the common axis with the outer annular portion 104a, and the three bridge portions coupled between the outer annular portion 104a and the inner annular portion 104b for resiliently supporting the inner annular portion 104b with respect to the outer annular portion 104a. Each of the bridge portions 104c has an arc shape and provided in the annular spacing 104f so as to extend along an inner periphery of the outer annular portion 104a and an outer periphery of the inner annular portion 104b. 
The bottom surface of the outer annular portion 104a of the upper leaf spring 104 is bonded to a top surface of the yoke 105 with a synthetic resin (adhesive) at several locations. The bonding operation is carried out by first applying the synthetic resin onto the top surface of the yoke 105, and then attach the upper leaf spring 104 onto the top surface of the yoke by pushing the upper leaf spring 104 against the yoke 105. The bonding portions by the synthetic resin are present between the upper leaf spring 104 and the top surface of the yoke 105, and they are indicated by the reference numerals 105a. 
It is to be noted that an actuator assembly similar to the actuator assembly 100 having such upper leaf spring 104 and lower leaf spring 110 described above is disclosed in JP-A-No. 2004-280031.
In the case where the upper leaf spring 104 is bonded to the yoke 105 with the synthetic resin as described above, there is a problem in that the synthetic resin at any one or more of the bonding portions is spread out due to the pressure applied when the upper leaf spring 104 is pushed against the top surface of the yoke 105 at the bonding operation. If the synthetic resin is spread out onto a base part 104d of the bridge portion 104c from the side of the outer annular portion 104a and then it is hardened on the base part 104d of the bridge portion 104, such a bridge portion 104c becomes difficult to be deformed desiredly, that is a spring constant of the bridge portion 104c is changed. As a result, a load characteristic of such a bridge portion 104c becomes different from those of the other bridge portions 104c, which results in the case that a stable load characteristic cannot be obtained in the upper leaf spring 104. If the upper leaf spring 104 does not have such a stable load characteristic, an initial response speed and an amount of an initial displacement of the holder 108 when an electrical current is supplied to the coil 107 become not stable, and thus it becomes difficult to precisely control the position of the holder 108.
In order to solve this problem, it may be contemplated to reduce an amount of the synthetic resin to be applied onto the top surface of the yoke 105. However, this approach in turn arises a new problem in that the upper leaf spring 104 may be partially peeled off from the top surface of the yoke due to the reduced bonding force in the event that the electronic apparatus equipped with the camera module is dropped by accident.
Further, in order to solve the problem as described above, it may be also contemplated to use a resistance welding method for attachment of the upper leaf spring 104 to the top surface of the yoke 150 without using any synthetic resin. However, in this method, it is necessary to apply a load to the upper leaf spring 104 placed on the top surface of the yoke 105. Due to this load, a stress deformation is likely to occur at the outer annular portion 104 of the upper leaf spring 104 as well as the base parts of the bridge portions 104c, which may result in variations in a load characteristic (spring constant) of the bridge portions 104c. Further, there is another problem in that, in the case where the resistance melding method is used, if the bottom surface of the outer annular portion 104c and/or the surface of the yoke 105 is oxidized, an electrical resistance at the resistance welding changes due to the oxidation, which may result in the case that a predetermined welding strength cannot be obtained.